1. Field of the Invention
The present invention relates to a molding composition for sealing carbon dioxide, and a molded composition having an excellent sealing property to be able to be used for sealing carbon dioxide. Also, the present invention relates to a sealing device to provide a seal between relatively movable portions on one of which a portion of the sealing device is held, sealingly contacting with another movable portion at a seal lip of the sealing device.
2. Related Art
Conventionally, Freon gas has been used in almost all automobile air-conditioners, refrigerators or the like as a coolant. But from the point of view of environment disruption (the depletion of the ozone layer, global warming or the like), new coolants other than Freon gas have been recently extensively studied. Carbon dioxide is considered to be one of a most hopeful next generation of coolants. In the case of conventional Freon gas, sealing materials such as hydrogenated NBR, EPDM (Ethylene-Propylene-Diene terpolymer) or the like have been used as sealing materials, but these materials have some defects such as swelling and blistering which occurs in the sealing materials when contacting carbon dioxide. These undesirable phenomena tend to cause the sealing device to be unusable. Particularly, the addition of a non-reinforcing filler with a large particle size such as mica or graphite for aiming to decrease in gas permeability of these rubber materials makes blisters occur easily, resulting in an unusable condition as a molded composition for sealing carbon dioxide.
On the other hand, there are some materials having a property of low gas permeability to carbon dioxide, such as vinylidene fluoride resin or the like. But these materials have a defect in processability or flexibility which rubber materials usually have. When these resin like materials are used as sealing materials--especially such a sealing means as a rotating shaft seal, a reciprocating shaft seal or the like, response to an axis to be sealed is slow when the axis is biased. This phenomena causes a microscopical gap between the seal lip and the axis and then to cause leakage of a sealing medium. In addition, when the sealing material made of vinylidene fluoride resin is assembled with an axis having splines (grooves made in the direction of the axis), a scratch is easy to occur by the splines and the scratch may not allow to keep optimum sealing performance.
There is a fluoro rubber which contains vinylidene fluoride. This type of fluoro rubber has excellent flexibility but the anti-permeability to carbon dioxide is not good.
Furthermore, it has been described in the publication of Japanese patent Tokkai Sho 59-40066 (1984), Tokkai Hei 6-25500 (1994), Tokko Hei 3-2197 (1991), Tokko Hei 3-59937 (1991) and Tokko Hei 7-68430 (1995) that a fluoro thermoplastic elastomer can be used for a molding material for a sealing composition resistant to lubricant. However, in these descriptions, MT carbon black (particle diameter: 201 to 500 nm) which is commonly used as a reinforcing filler for fluoro rubbers has been used as it is.
When fluoro thermoplastic elastomer containing MT carbon black is used for sealing devices for sealing carbon dioxide, both permeability of and swelling due to exposure to carbon dioxide is large, and a blister phenomena can be seen so that these compositions cannot be safely used in the seal for sealing carbon dioxide.